linux-kernel - Re: [PATCH] mm: nobootmem: avoid type warning about alignment value

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Message-ID: <20131210005454.GX4360@n2100.arm.linux.org.uk>
Date:	Tue, 10 Dec 2013 00:54:54 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Santosh Shilimkar <santosh.shilimkar@...com>,
	Tejun Heo <tj@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Sergei Shtylyov <sergei.shtylyov@...entembedded.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] mm: nobootmem: avoid type warning about alignment value

On Mon, Dec 09, 2013 at 04:50:44PM -0800, Andrew Morton wrote:
> On Mon, 25 Nov 2013 08:57:54 -0500 Santosh Shilimkar <santosh.shilimkar@...com> wrote:
> 
> > On Sunday 24 November 2013 10:14 AM, Sergei Shtylyov wrote:
> > > Hello.
> > > 
> > > On 24-11-2013 3:28, Santosh Shilimkar wrote:
> > > 
> > >> Building ARM with NO_BOOTMEM generates below warning. Using min_t
> > > 
> > >    Where is that below? :-)
> > > 
> > Damn.. Posted a wrong version of the patch ;-(
> > Here is the one with warning message included.
> > 
> > >From 571dfdf4cf8ac7dfd50bd9b7519717c42824f1c3 Mon Sep 17 00:00:00 2001
> > From: Santosh Shilimkar <santosh.shilimkar@...com>
> > Date: Sat, 23 Nov 2013 18:16:50 -0500
> > Subject: [PATCH] mm: nobootmem: avoid type warning about alignment value
> > 
> > Building ARM with NO_BOOTMEM generates below warning.
> > 
> > mm/nobootmem.c: In function _____free_pages_memory___:
> > mm/nobootmem.c:88:11: warning: comparison of distinct pointer types lacks a cast
> > 
> > Using min_t to find the correct alignment avoids the warning.
> > 
> > Cc: Tejun Heo <tj@...nel.org>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Signed-off-by: Santosh Shilimkar <santosh.shilimkar@...com>
> > ---
> >  mm/nobootmem.c |    2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/mm/nobootmem.c b/mm/nobootmem.c
> > index 2c254d3..8954e43 100644
> > --- a/mm/nobootmem.c
> > +++ b/mm/nobootmem.c
> > @@ -85,7 +85,7 @@ static void __init __free_pages_memory(unsigned long start, unsigned long end)
> >  	int order;
> >  
> >  	while (start < end) {
> > -		order = min(MAX_ORDER - 1UL, __ffs(start));
> > +		order = min_t(size_t, MAX_ORDER - 1UL, __ffs(start));
> >  
> 
> size_t makes no sense.  Neither `order', `MAX_ORDER', 1UL nor __ffs()
> have that type.
> 
> min() warnings often indicate that the chosen types are inappropriate,
> and suppressing them with min_t() should be a last resort.
> 
> MAX_ORDER-1UL has type `unsigned long' (yes?) and __ffs() should return
> unsigned long (except arch/arc which decided to be different).
> 
> Why does it warn?  What's the underlying reason?

The underlying reason is that - as I've already explained - ARM's __ffs()
differs from other architectures in that it ends up being an int, whereas
almost everyone else is unsigned long.

The fix is to fix ARMs __ffs() to conform to other architectures.
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